[Ayoka]

It sounds like he was really scared...is there any chance that someone could have hit him or hurt him while you weren't around. I'm sure that could make him afraid of strangers all of a sudden.

Or maybe the two strangers were wearing hats, scarves or the older man had a cane that spooked him.

It does sound bizarre to me that a normally outgoing pup would all of a sudden be afraid of strangers.